The present invention relates to the optimal operation of a slurry bubble column reactor. Such columns have three phases in which solid catalyst particles are held in suspension in a liquid phase by bubbling gas phase reactants.
Slurry bubble column reactors operate by suspending catalytic particles in a liquid and feeding gas phase reactants into the bottom of the reactor through a gas distributor which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalytic system, they can be converted to both liquid and gaseous products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered by passing the slurry through a filter which separates the liquid from the catalytic solids. A principal advantage of slurry reactors over fixed bed reactors is that the presence of a circulating/agitated slurry phase greatly increases the transfer rate of heat to cooling surfaces built into the reactor. Because reactions of interest are often highly exothermic, this results in reduced reactor cost (less heat transfer equipment is needed) and improved stability during reactor operations. A distinct advantage of bubble columns over mechanically stirred reactors is that the required mixing is effected by the action of rising bubbles, a process significantly more energy-efficient than mechanical stirring.
In any reaction, the rate of conversion of reactants to products and the product selectivity depend on the partial pressure of the reactants in contact with the catalyst. Thus, the mixing characteristics of the reactor become critical in determining catalyst performance because they will determine the gas phase composition (and therefore, the partial pressure of the reactants) at any particular axial position in the reactor.
In fully backmixed reactors (CSTR), the composition of reactants (gaseous) and products (liquids and gases) and condition of the catalyst is identical at every point within the reactor. The reactant concentration (or gas partial pressure) controls catalyst performance by providing the driving force for the reaction and determines the conversion occurring in the reactor. Thus, even though pure reactant feed is entering the reactor, catalyst performance is driven by the uniform reactant gas phase concentration present throughout the reactor and equal to the reactant gas phase concentration exiting the reactor. This fully backmixed system has a low relative productivity per volume of reactor for any reactions having positive pressure order rate kinetics.
The other extreme in reactor mixing occurs in plug flow reactors where the catalyst is stationary relative to the flow of reactants and products (liquids and gases). The feed undergoes reaction as it enters the reactor and the reaction continues as the unreacted feed proceeds through the reactor. Thus, the concentration and partial pressure of reactants decrease along the path of the reactor; therefore, the driving force of the reaction also decreases as the concentration of liquid and gaseous products increase. Thus, the catalyst at the exit portion of the plug flow reactor never sees fresh feed. The plug flow system provides maximum productivity for a given reactor volume for any reactions showing positive pressure order kinetics.
The important difference between the CSTR and plug flow reactor systems is that the gas phase reactant concentrations that provide the kinetic driving force for the reaction differ significantly. In the fully backmixed system, the reactant concentration is the same at every point in the reactor; in the plug flow system, the reactant concentration steadily decreases along the path of the catalyst bed from inlet to outlet and the reaction rate is obtained by integrating the rate function from inlet to outlet. Because the reactant concentration at any point in a CSTR system always corresponds to outlet conditions, the productivity in a fully backmixed system will always be lower than the productivity in a plug-flow system for reactions with positive pressure order kinetics.
Reactor systems exhibiting plug-flow and well stirred characteristics represent extremes in reactor performance. In practice, plug-flow reactors may exhibit some backmixed traits and backmixed reactors may exhibit some plug-flow traits. Deviations from the ideal systems are due to the dispersion of the reactant gases in the reactor. Extent of backmixing is a function of the mechanical energy imparted to the system. The reactor geometry also affects backmixing and small L/d (i.e., reactor length to reactor diameter) ratios, less than 3, favor complete backmixing. However, higher energy input reactors with greater L/d can also achieve complete backmixing. Conversely, plug-flow behavior is favored by high L/d ratios. The degree of backmixing that can occur in a plug-flow reactor can be represented by the Peclet number, Pe. (See Carberry, J. J., "Chemical and Catalytic Reaction Engineering", McGraw-Hill, 1976, or Levenspiel, O., "Chemical Reaction Engineering", Wiley, 1972.)
High Peclet numbers, e.g., greater than 10, lead to plug-flow behavior while low Peclet numbers, e.g., less than 1, correspond to well-mixed systems and are typical of CSTR's. By definition, the dispersion coefficient for an ideal CSTR is infinity and the Peeler number approaches zero.
These considerations show that the scale-up of slurry reactors from laboratory to commercial units is not straightforward. For example, as the reactor vessel is made taller, the height to which the catalyst is fluidized is likely not to increase proportionally or at all, and the added reactor volume remains unused. Also, as the reactor diameter increases, the mixing intensity increases and may result in an increase in the fluidization height but could also increase the Peclet number and move the reactor performance from plug-flow to well-mixed with a corresponding decrease in conversion of products to reactants.
This difficulty is obvious in previous attempts to apply slurry reactors to the important process of Fischer-Tropsch synthesis of hydrocarbons (predominantly C.sub.10 +) from synthesis gas (carbon monoxide and hydrogen) using iron catalysts. The sole previous scale-up efforts reported in the literature for commercial size units (5 ft. diameter) were the Rheinpreussen tests in the 1950's (see H. Storch, N. Columbis, R. B. Anderson, "Fischer-Tropsch and Related Synthesis", Wiley (1951) New York and J. Falbe, "Advances in Fischer-Tropsch Catalysis", Verlag (1977) Berlin) . Moving from laboratory to commercial units, they sequentially built systems in which the dispersion was too low to adequately fluidize the particles to systems with dispersions high enough to cause backmixed reactor behavior in the commercial size reactor. To date, the optimal implementation of large scale systems has not been achieved or described. A methodology for such a scale-up process is described in this invention.
Optimum performance of slurry bubble column reactors requires adequate fluidization of the catalyst particles while minimizing backmixing of the reactants in the gas phase. If the conditions in the reactor are such that the particles settle, difficulties arise because the reaction zone is short. Then in order to achieve high conversions, the reaction rate per volume must be very high and the catalyst can easily become starved of reactants because of limitations in the rate at which reactants can be transferred from the gas bubbles to the particles suspended in the liquid. This condition results in poor catalyst utilization, poor reaction selectivity, and eventually to catalyst deactivation. Also, for exothermic reactions, the heat release takes place in the short reaction zone, imposing severe requirements on the heat transfer equipment.
The tendency of the particles to settle can be overcome, however, by maximizing the dispersion effects resulting from the rising gas bubbles and from the mixing patterns that they induce. These dispersion effects can be enhanced by increasing either the effective reactor diameter or the flow rate of gas through the reactor. If the dispersion is increased too much, however, the gas phase will also become well mixed and the reactor performance will change from that of a plug flow reactor to that of a backmixed reactor.
Eri et al in U.S. Pat. No. 4,857,559 have discussed the relative merits of operating a Fischer-Tropsch reactor with a feed gas containing various levels of diluents such as methane, carbon dioxide, and nitrogen. In fixed bed reactors, they have indicated that the presence of a diluent such as nitrogen in the feed is disadvantageous since it will increase the pressure drop across the reactor bed. In a slurry or fluidized bed reactor they indicate that diluent has beneficial effects, in that it provides additional mixing energy to the system to keep the catalyst suspended. Moreover they note that added diluent will not have a great effect on pressure drop in the slurry or fluidized bed reactors.
Eri et al also indicated that diluents will have a disadvantageous effect on the fixed bed reactor since it will, at constant overall pressure, lead to a net reduction in the partial pressure of reactant gases present with a concomitant net reduction in the overall volumetric productivity of the system. They failed to indicate, however, that a similar reduction in productivity would result in slurry or fluidized bed reactors as the diluent reduces the reactant gas partial pressure. Consequently, the improved catalyst fluidization achieved with added diluent is offset by the reduced productivity and subsequent diluent processing steps associated with product recovery.
The preferred embodiment of the present invention is the Fischer-Tropsch synthesis of hydrocarbons using CO catalysts. The Fischer-Tropsch reaction involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to higher aliphatic alcohols, The methanation reaction was first described by Sabatier and Senderens in 1902. The later work by Fischer and Tropsch dealing with higher hydrocarbon synthesis (HCS) was described in Brennstoff-Chem, 7, 97 (1926).
The reaction is highly exothermic and care must be taken to design reactors for adequate heat exchange capacity as well as for their the ability to continuously produce and remove the desired range of hydrocarbon products. The process has been considered for the conversion of carbonaceous feedstocks, e.g., coal or natural gas, to higher value liquid fuel or petrochemicals. The first major commercial use of the Fischer-Tropsch process was in Germany during the 1930's. More than 10,000 B/D (barrells per day) of products were manufactured with a cobalt based catalyst in a fixed-bed reactor. This work has been described by Fischer and Pichler in Ger. Pat. No. 731,295 issued Aug. 2, 1936.
Commercial practice of the Fischer-Tropsch process has continued in South Africa in the SASOL plants. These plants use iron based catalysts and produce gasoline in fluid-bed reactor and wax in fixed-bed reactors.
Research aimed at the development of more efficient CO hydrogenation catalysts and reactor systems is continuing. In particular, a number of studies describe the behavior of iron, cobalt or ruthenium based catalysts in slurry reactors together with the development of catalyst compositions and improved pretreatment methods specifically tailored for that mode of operation.
Farley et al in The Institute of Petroleum, vol. 50, No. 482, pp. 27-46, February (1984), describe the design and operation of a pilot-scale slurry reactor for hydrocarbon synthesis. Their catalysts consisted of precipitated iron oxide incorporating small amounts of potassium and copper oxides as promoters. These catalysts underwent both chemical and physical changes during activation with synthesis gas in the slurry reactor.
Slegeir et al in Prepr. ACS Div. Fuel Chem, vol. 27, p. 157-163 (1982), describe the use of supported cobalt catalysts for the production of hydrocarbons from synthesis gas at pressures above 500 psi in a CSTR slurry reactor.
Rice et al in U.S. Pat. No. 4,659,681 issued on Apr. 21, 1987, describe the laser synthesis of iron based catalyst particles in the 1-100 micron particle size range for use in a slurry Fischer-Tropsch reactor.
Dyer et al in U.S. Pat. No. 4,619,910 issued on Oct. 28, 1986, and U.S. Pat. No. 4,670,472 issued on Jun. 2, 1987, and U.S. Pat. No. 4,681,867 issued on Jul. 21, 1987, describe a series of catalysts for use in a slurry Fischer-Tropsch process in which synthesis gas is selectively converted to higher hydrocarbons of relatively narrow carbon number range. Reactions of the catalyst with air and water and calcination are specifically avoided in the catalyst preparation procedure. Their catalysts are activated in a fixed-bed reactor by reaction with CO+H.sub.2 prior to slurrying in the oil phase in the absence of air.
Fujimoto et al in Bull. Chem. Soc. Japan, vol. 60, pp. 2237-2243 (1987), discuss the behavior of supported ruthenium catalysts in slurry Fischer-Tropsch synthesis. They indicate that the catalyst precursors were ground to fine powders (&lt;150 mesh), calcined if needed, and then activated in flowing hydrogen before addition to a degassed solvent and subsequent introduction to the slurry reactor.